Companion planting, is it just tosh or is there something in it?

By all means mix plants together and avoid monoculture but read on...

It is all very complex
Companion planting is usually associated with pest control that arises from planting two or more different plant subjects together. The concept can be extended to the idea of plants being mutually beneficial to each other in other ways such as combined yield increase or giving each other a protected environment.
Both ideas can be flawed and in particular the first concept is supported by little evidence, gross exaggeration and wishful thinking and endlessly repeated myths such that companion plants need to be strongly scented. 
Except there are exceptions. Nothing in the fascinating plant world is simple.

As to the second concept, plant yield is usually limited by mutual competition for such as  available light and water. Growing for example a root crop among a leaf crop leads to lower combined yields than when the plants were grown separately.

I suggest that most companion planting arising from dedicated pairings or perhaps more commonly a ‘deterrent plant’ within a crop are a waste of time either because they are  completely ineffective or have only marginal impact which in normal practice is useless.

Take the highly popular belief that marigolds deter whitefly on such as tomatoes. Well they actually do - but not well enough. For me one whitefly is too many - on a plant like tomato which is so susceptible to glasshouse whitefly (indoors and out) a single whitefly soon becomes an infestation. Those who declare complete success with marigolds as a deterrent are probably good growers who do not insist on overwintering whitefly in their heated greenhouses on such as pelargoniums and do not introduce it into their gardens in the first place in Summer on such as their greenhouse propagated french beans. Without any companion planting I have not suffered from whitefly in twenty years. I would go so far to say that  my tomatoes have not encountered a single whitefly in that time (As opposed to brassica whitefly on my sprouts which is a different story)

You might be wondering why I have apparently destroyed my case by quoting one of the very few pairings, whitefly and marigolds where there is actually some deterrent effect! Indeed Newcastle University have extracted limonace (also the main oil in orange peel), the chemical actually responsible and have suggested it might be applied as a spray. To my mind spraying would be the kiss of death to the idea bearing in mind how gardeners seem to regard any spray as the work of the devil!

My example stands out as a kind of success story among other rather more useless popular pairings and my point is that a gardener has better things do than companion planting - except he might get pleasure from his illusions and might even like marigolds in his carrots. Actually marigolds are a fascinating plant and do produce root secretions toxic to eelworms and for certain plants in certain climates they might be useful - but in your own garden? Forget it.

I remember a bright intelligent student totally imbued with organic notions. He enlivened my lectures - not an easy task. His ‘plot project’ endeavoured to demonstrate the success of companion planting. With the best will in the the world we could not discern the benefits he purported to show. (But he won the plot prize)

I am indebted to Robert Pavlis whose latest post shines further light on companion planting. He shares much of my scepticism  and his post specifically rubbishes the idea that it is scented plants that work these pest deterrent wonders. But first read this…

Evolved relationships between pathogens and plants
Is companion planting a waste of time?
Gardeners may feel that modern research which shows how plants communicate one with another -  often by scent - gives strength to the belief that scented plants confuse pest predators in companion planting. In fact it is very specific pheromones by which plants can communicate in closely evolved relationships where for example they warn one another about the presence of pests and consequently produce chemicals antagonistic to them. It does not follow that because plants have a strong scent they do the same. Our own senses do not discern a  plant’s subtle communications.
You might imagine that measures of natural plant self protection bear some resemblance to precise controls required by the gardener. Not so, there are subtle balances between potential pathogen and host. After all plant priority is to set a lot of fertile seed. It is not a priority to kill every last pest - that is too expensive to the plant’s resources.

To go further when you look at the plant world there is a spectrum between plants and potential predators which runs between complete symbiosis to total destruction. It is sometimes not clear whether an organism is an enemy or friend.
Why is it that such pests as red spider mite or whitefly cosy up to certain plants in abundance and yet other plants hardly suffer from them at all. Indeed one researcher asked why is it that most pests have restricted hosts and only attack vulnerable species? Is there anything in it for the so called victim?
It can all be represented as evolutionary combat.

It might be taking things a bit far to speculate why a plant might allow itself to tolerate a particular pest. A researcher suggested that aphids could be good for a plant in the wild that is in infertile conditions. Aphids drop honeydew which can stimulate nitrogen fixation in the soil. The plant effectively gives up sugar to achieve more nitrogen and thereby maximise its seed production.

My Solomon's seal missed the sawfly this year

I myself wonder why solomon's seal allows sawfly to decimate it so severely year upon year? Could it be that in late Spring it has completed its life cycle and hived away in its thick roots ample resources? It just might be a service for the tough leathery leaves to be chomped up into manure.

Why do brussel sprouts abandon their green leaves on old stems when attacked severely by fungus disease in late Autumn and yet the sprouts and the plant tops stay green? Is it saving its resources to fight the real battle?

You might very well ask what is all this to do with companion planting? I am not really sure myself! It does illustrate that plant/ pest relations are complicated and most companion plantings are too simplistic and that nature’s aims are not the same as our own.

(If at this point I might put in a promotion for my next but one post. It is about symbiosis when organisms  evolve in harmony. New thinking is that symbiosis is a prime driver of evolution right down to genome level. Those of you have followed my series on evolution might wish to read it)

The appropriate/inappropriate pest landing theory
Thank you Robert Pavlis for reporting this elegant study at Horticulture Research International, Wellsbourne UK . I applaud this prestigious institute which in a previous guise was the National Vegetable Research Station. If you grow your vegetables in anything like a modern away you can be sure that their innovations play a major part.
In a series of elegant experiments they demonstrated that flying vegetable pests having generally identified the area of their host crop by scent from the air and are only attracted down to the ground by the colour green. Any green plant would do and even green coloured paper attracted the pest. They planted a wide range of potential companion plants around vegetables crops and all that were green attracted the pest. Any plants that were any other colour did not attract the pest.
The pest detects on alighting whether anything green is the appropriate host. If not it tries again on a nearby green plant. The more ‘false starts’ the less eggs will be laid. It was shown that any green ‘companion plant’ fulfilled this function. Conventional coloured scented ‘herbs’ performed poorly. There were several examples of vegetables that had lower pest loads when they had  green companions such as clover!

This might lead you to the correct conclusion that such as ground cover and weeds(!) would both satisfy this deterrent effect!
Hang on! Weeds might decimate your crop far more than pests and ground cover might reduce yields by competing for water. Take care not to draw the wrong conclusions from new discoveries!

Companion plants as a lure
I knew a dutch commercial grower of tomatoes who used a few pots of french beans in his glasshouse as a trap for whitefly. Each week he would remove the infected pots and replace them.
I have not time to discuss this principle today but will merely quote Robert Pavlis’s dry comment that his neighbours are only too welcome to plant them

Grow a wide range plants and avoid monoculture
Although I have been sceptical today about targeted companion planting I must hold true to my conviction that growing a wide range of plants in your garden is a fundamental arm of cultural pest control where diversity provides a myriad of habitats for predators of pests. In contrast farmers and commercial growers are stuck with large monocultures and find the need to regularly spray

Links

Robert Pavlis writes about companion planting

Peter William's  article on natural planting